Backreaction in Axion Monodromy, 4-forms and the Swampland

TL;DR

The paper analyzes backreaction in axion monodromy models by recasting the problem in terms of Minkowski 4-forms, where saxion-dependent kinetic metrics capture how backreaction alters the inflaton's field range. It shows a universal logarithmic scaling of the proper field distance at large field values, consistent with Swampland expectations, and demonstrates that in typical Type IIA flux compactifications the critical distance is flux-independent and near the Planck scale. It also investigates whether a mass hierarchy (e.g., via D7-brane moduli in Type IIB) can delay backreaction, finding that in realistic global constructions such tuning is not achievable without violating EFT validity. Overall, the results support the Refined Swampland Conjecture, while highlighting potential model-building avenues (e.g., specific D7-brane scenarios) that merit further study to assess their viability in the string landscape.

Abstract

Axion monodromy models can always be described in terms of an axion coupled to 3-form gauge fields with non-canonical kinetic terms. The presence of the saxions parametrising the kinetic metrics of the 3-form fields leads to backreaction effects in the inflationary dynamics. We review the case in which saxions backreact on the Kähler metric of the inflaton leading to a logarithmic scaling of the proper field distance at large field. This behaviour is universal in Type II string flux compactifications and consistent with a refinement of the Swampland Conjecture. The critical point at which this behaviour appears depends on the mass hierarchy between the inflaton and the saxions. However, in tractable compactifications, such a hierarchy cannot be realised without leaving the regime of validity of the effective theory, disfavouring transplanckian excursions in string theory.